Reaction mechanism for booster

ABSTRACT

In accordance with the invention, a booster is not provided with a mechanism which transmits a braking reaction, and accordingly, a reaction cannot be transmitted to a brake pedal. On the other hand, a pseudo-reaction imparting means is provided to impart a pseudo-reaction which depends on an amount of depression of the brake pedal. The pseudo-reaction imparting means imparts a pseudo-reaction having a reduced rate of increase to the brake pedal when a travel of the brake pedal is small, and imparts a pseudo-reaction having an increased rate of increase to the brake pedal when a travel of the break pedal is higher. This arrangement avoids the drawback of the prior art that an abnormally high braking reaction is transmitted to a driver as a result of an operational lag of a conventional booster during a quick braking operation. The invention prevents such braking reaction from being transmitted to a driver, but enables a reaction of a required magnitude to be transmitted to the driver by means of pseudo-reaction imparting means. In this manner, a required high braking effort can be obtained with a reduced force of depression during a quick braking operation. The pseudo-reaction imparting means which exhibit such characteristic is capable of realizing an operation feeling, as experienced in a normal operation in the prior art, independently from a quick and a slow braking operation.

This is a division of Ser. No. 08/859 739, filed May 21, 1997 now U.S.Pat. No. 6,003,426.

FIELD OF THE INVENTION

The invention relates to a booster as may be used in a brake of anautomobile, and more particularly, to a reaction mechanism for boosterwhich imparts reaction force to an operating rod which actuates thebooster.

DESCRIPTION OF THE PRIOR ART

A brake booster is known in the art which comprises a valve bodyslidably disposed within a shell, a power piston mounted on the valvebody, a constant and a variable pressure chamber defined across thepower piston, a valve mechanism disposed in the valve body, an inputshaft connected to a brake pedal serving as an operating rod and adaptedto drive a valve plunger, which forms part of the valve mechanism, backand forth to thereby switch a flow path in the valve mechanism, and anoutput shaft driven forward as the valve body is driven forward to causea piston of a master cylinder to be driven forward, thereby developing abraking liquid pressure.

A conventional reaction mechanism is provided with a reaction discformed of rubber and disposed between the output shaft and the valveplunger so that when the brake booster is actuated, both the valve bodyand the valve plunger are brought into contact with the reaction discsimultaneously, part of a braking force applied to the output shaftbeing transmitted to the valve body while the remainder thereof istransmitted to the valve plunger, the braking reaction force transmittedto the valve plunger being sensed by a driver through the input shaftand the brake pedal.

A serve ratio of the brake booster can be changed by changing the ratioof the reaction force transmitted to the valve body and the reactionforce transmitted to the valve plunger, and more specifically, the ratioof pressure-responsive areas of the both members. A high serve ratio isgenerally established in the brake booster so that a braking liquidpressure of an increased magnitude can be developed with a depression ofthe brake pedal with a reduced force. However, it is found that during aquick braking operation, a serve ratio of a predetermined magnitudecannot be obtained as a result of an operational lag of the brakebooster, and it is difficult for a less powerful driver such as an agedperson or woman to perform a quick braking operation.

More specifically describing the reason, when the brake pedal isdepressed, a flow path in the valve mechanism is switched through theinput shaft, whereby a pressure fluid is introduced into the variablepressure chamber to drive the power piston and the valve body forward.As the valve body is driven forward, the output shaft is also drivenforward through the reaction disc, the forward movement of the outputshaft developing a braking pressure. A reaction therefrom is applied tothe output shaft, and the brake reaction applied to the output shaft isdistributed between the valve body and the valve plunger in a mannermentioned above. However, the valve plunger which is connected to thebrake pedal through the input shaft will be driven forward before thepower piston and the valve body are driven forward by the pressure fluidwhich is introduced into the variable pressure chamber during a quickbraking operation, and as a consequence, a major proportion of thebraking reaction which is applied to the output shaft will betransmitted to the valve plunger. This results in a braking reaction ofan abnormally high magnitude being transmitted to the driver.Consequently, when a quick braking operation is to be performed, thebrake pedal must be depressed by overcoming the abnormal brakingreaction which is transmitted. As compared with a normal brakingoperation in which the brake pedal is gradually depressed to achieve ahigh braking force, a braking force of an increased magnitude which isrequired for a quick braking operation cannot be obtained unless thebrake pedal is depressed with a force which is much greater than in anormal braking operation.

SUMMARY OF THE INVENTION

In view of the foregoing, the invention provides a reaction mechanismfor booster which enables a high output to be obtained with a reducedforce of depression during a quick braking operation without degrading anormal operation feeling of the booster.

Thus, the invention relates to a booster including a valve body slidablydisposed within a shell, a power piston mounted on the valve body, aconstant and a variable pressure chamber defined across the powerpiston, a valve mechanism disposed in a valve body, an input shaftconnected to an operating rod for driving a valve plunger, which forms avalve mechanism, back and forth to switch a flow path in the valvemechanism, and an output shaft which is driven forward as the valve bodyis driven forward. In accordance with the invention, there is providedpseudo-reaction imparting means which prevents the reaction applied tothe output shaft during the operation of the booster from beingtransmitted to the valve plunger and which imparts a pseudo-reactionwhich depends on a travel of the input shaft to the operating rod. Thepseudo-reaction imparting means is arranged such that a pseudo-reactionhaving a lower rate of increase is imparted to the operating rod when atravel of the operating rod is low, while a pseudo-reaction having ahigher rate of increase is imparted to the operating rod when a travelof the operating rod is higher.

With the described arrangement, upon actuation of the booster, thereaction applied to the output shaft is prevented from being transmittedto the valve plunger, and accordingly, during a quick braking operation,if the valve plunger which is connected to the operating rod through theinput shaft is driven forward before the power piston and the valve bodyare driven forward by pressure fluid introduced in the variable pressurechamber, the transmission of a reaction applied to the output shaft to adriver through a path including the valve plunger, the input shaft andthe operating rod is prevented.

On the other hand, the pseudo-reaction imparting means transmits apseudo-reaction which depends on a travel of the operating rod to thedriver through the operating rod. In this manner, the transmission of anabnormally high reaction during a quick braking operation is prevented,and instead, a high output can be obtained with a reduced operatingforce in the same manner as in a usual operation.

As mentioned above, the pseudo-reaction imparting means imparts apseudo-reaction having a lower rate of increase to the operating rodwhen a travel of the operating rod is low, and imparts a pseudo-reactionhaving an increased rate of increase to the operating rod when a travelof the operating rod is high. Accordingly, a similar operation feelingas in a usual operation can be obtained independently from a quick and aslow braking operation. By way of example, when an operating rodcomprises a brake pedal, during an initial phase of the brakingoperation, an increase in the pseudo-reaction will be low if the brakepedal is depressed through an increased stroke, but after a brakingforce of certain magnitude is developed, the magnitude ofpseudo-reaction will increase relative to the amount of depression ofthe brake pedal if it is small. The pseudo-reaction imparting meanswhich exhibits such characteristic is effective in realizing such anormal operation feeling.

Above and other objects, features and advantages of the invention willbecome apparent from the following description with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a first embodiment of the invention;

FIG. 2 is a cross section of a second embodiment of the invention;

FIG. 3 is a cross section of a third embodiment of the invention;

FIG. 4 is a cross section of a fourth embodiment of the invention;

FIG. 5 is an enlarged view of part shown in FIG. 4;

FIG. 6 is a cross section taken along the line V—V shown in FIG. 5;

FIG. 7 graphically shows the characteristic diagram of a fourthembodiment;

FIG. 8 is a schematic view of a fifth embodiment of the invention;

FIG. 9 is a front view of a sixth embodiment of the invention; and

FIG. 10 is a side elevation of FIG. 9.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to the drawings, several embodiments of a brake booster towhich the invention is applied will be described. Referring to FIG. 1, afront shell 1 and a rear shell 2 constitute together an enclosed vessel,in which a substantially tubular valve body 3 is slidably disposed. Apower piston 4 is mounted around the outer periphery of the valve body 3and a diaphragm 5 is applied to the rear side thereof, thus partitioningthe interior of the enclosed vessel into a forwardly located constantpressure chamber 6 and a rearwardly located variable pressure chamber 7.

The valve body 3 contains a valve mechanism 9 which switches a fluidcircuit. Specifically, the valve mechanism 9 comprises an annular firstvalve seat 10 formed around the inner periphery of the valve body 3, anannular second valve seat 12 formed on the right end of a valve plunger11 which is slidably fitted in the valve body 3, and a valve element 14which is urged from the right, as viewed in FIG. 1, to be seated uponeither valve seat 10 or 12 under the influence of a poppet return spring13.

A space located radially outward of the first valve seat 10 communicateswith the constant pressure chamber 6 through an axial constant pressurepassage 15 formed in the valve body 3, and the constant pressure chamber6 communicates with an intake manifold of an engine through a tubing 16mounted on the front shell 1 for introducing a negative pressure. Inthis manner, a negative pressure is normally introduced into the chamber6. A space located intermediate the first valve seat 10 and the secondvalve seat 12 communicates with the variable pressure chamber 7 througha radial variable pressure passage 17 formed in the valve body 3.Finally, a space located radially inward of the second valve seat 12communicates with the atmosphere through a pressure passage 18 formed inthe valve body 3.

The right end of the valve plunger 11 is pivotally connected with adistal end of the input shaft 21, the other end of which is connected toa brake pedal (operating rod), not shown. The plunger 11 is preventedfrom being disengaged from the valve body 3 by means of a key member 22.While not shown, the key member 22 is forked in a region from its centertoward the distal end thereof, and the key member 22 is inserted into areceiving hole 23 which is diametrically formed in the valve body 3,with the end from which the forked portion extends is engaged with aportion 11 a of the valve plunger which has a reduced diameter.

It is to be noted that the key member 22 is displaceable axially of thevalve body 3 within the hole 23, and the key member 22 and the valveplunger 11 are displaceable axially of the valve body 3 within an extentof the axial length of the portion 11 a. When the brake booster isinoperative, the key member 22 is held in abutment against the innersurface of the rear shell 2 to maintain the key member 22 and the valveplunger 11 at an advanced position relative to the valve body 3 so thata lost motion of the input shaft 21 at the commencement of operation ofthe brake booster can be reduced.

Formed in the right end of an output shaft 14 and which is disposed tothe left of the valve plunger 11 is a recess 24 a in which an elasticmember 25 formed of rubber and a spacer 26 formed of a metal disc aresequentially fitted, and an annular projection 3 a formed to extend fromthe right end of the valve body 3 is also slidably fitted in the recess24 a.

The left or the distal end of the output shaft 24 projects externally ofthe front shell 1 with a seal 27 maintaining a hermetic seal, and thedistal end is connected to a piston of a master cylinder, 24 b, which isconnected to the front shell 1. It is to be noted that the valve body 3and the power piston 4 are normally maintained in the inoperativeposition shown by a return spring 28.

It is to be understood that in a conventional practice, the spacer 26has been omitted, and the left end face of the valve plunger 11 isdisposed in opposing relationship with the elastic member 25, serving asa reaction disc, with a very slight clearance therebetween so that whenthe brake booster is actuated, the brake reaction transmitted from theoutput shaft 24 can be transmitted to both the valve body 3 and thevalve plunger 11 through the elastic member 25, and the braking reactiontransmitted to the valve plunger 11 is transmitted to a driver inputthrough the input shaft 21 and a brake pedal (operating rod), not shown.

By contrast, in the present embodiment, the spacer 26 is included, andan increased spacing is provided between the spacer 26 and the valveplunger 11 so that an abutment therebetween can be avoided even during aquick braking operation. In this manner, the entire braking reactionwhich is transmitted from the output shaft 24 is transmitted to thevalve body 3 through the elastic member 25 and the spacer 26 while nobraking reaction is transmitted to the valve plunger 11, and hence tothe brake pedal. In the present embodiment, the elastic member 25 isused as an impact buffering member.

The described arrangement alone cannot provide a brake feeling to adriver. Accordingly, a pseudo-reaction imparting means 31 is provided toimpart a pseudo-reaction which depends on an amount of depression of thebrake pedal to a driver.

The pseudo-reaction imparting means 31 utilizes a valve return spring 32which urges the input shaft 21 rearwardly. In the conventional practice,a valve return spring is disposed between the valve body 3 and the inputshaft 21, but in the present embodiment, it is disposed between atubular member 33 which is integral with the carrosserie (a part of acar body) and the input shaft 21.

The tubular member 33 comprises an outer tubular portion 33 a, an innertubular portion 33 b which is inserted into the valve body 3 from theright side thereof, a connector portion 33 c which joins both tubularportions 33 a, 33 b at their right ends, and a flange 33 d whichprojects radially inward from the left end of the inner tubular portion33 b. The rear shell 2 includes, at its center, a tubular portion 2 awhich extends rearwardly and which is disposed as a press fit within theouter tubular portion 33 a while the left end face of the outer tubularportion 33 a is held in abutment against the outer surface of the rearshell 2 simultaneously, thus integrally connecting the tubular members33 and the rear shell 2 together. The rear shell 2 is mounted on thecarrosserie, not shown, by means of a mounting bolt 34 shown mountedthereon, and therefore it will be seen that the tubular member 33 isfixedly mounted on the carrosserie through the rear shell 2. A retainer35 is fixedly mounted on the input shaft 21, and the valve return spring32 is disposed between the flange 33 d of the tubular member 33 and theretainer 35 to urge the input shaft 21 rearward.

With the described arrangement, when the brake pedal is depressed todrive the input shaft 21 and the valve plunger 11 forward, a flow pathwithin the valve mechanism 9 is switched to introduce the atmosphereinto the variable pressure chamber 7 in the similar manner as in aconventional brake booster. Accordingly, a pressure differential betweenthe constant pressure chamber 6 and the variable pressure chamber 7 iseffective to drive the power piston 4 and the valve body 3 forward, thuscausing the output shaft 24 to be driven forward to develop a brakingliquid pressure in a master cylinder.

A braking reaction which is produced by the braking liquid pressure isentirely transmitted to the valve body 3 through the output shaft 24,the elastic member 25 and the spacer 26, and no portion thereof istransmitted to the valve plunger 11.

On the other hand, when the brake pedal is depressed to drive the inputshaft 21 forward, it will be seen that the input shaft 21 moves forwardwhile compressing the valve return spring 32 against the flange 33 d ofthe tubular member 33 which is fixedly mounted on the carrosserie, andaccordingly, there results a pseudo-reaction which is proportional tothe compression of the valve return spring 32. This pseudo-reaction istransmitted to a driver through the input shaft 21 and the brake pedal.Accordingly, by choosing an optimum condition for a preset load, presetlength or spring constant of the valve return spring 32, there can beobtained an excellent brake feeling comparable to the prior art for thedriver.

It will be seen that the pseudo-reaction imparting means 32 imparts apseudo-reaction to the brake pedal, which remains substantially the sameduring a quick braking operation as in a normal braking operation, thusavoiding the occurrence of an abnormally high braking reaction asexperienced with a conventional arrangement during a quick brakingoperation. In this manner, a high braking output can be obtained with arelatively low force of depression during a quick braking operation, anda reliable quick braking operation can be assured for a powerless driversuch as an aged person or woman.

It is to be noted that the elastic member 25 and the spacer 26 may beomitted.

Second Embodiment

FIG. 2 shows a second embodiment of the invention in which apseudo-reaction imparting means 131 is contained within the brakebooster. It should be noted that parts and members corresponding tothose shown in FIG. 1 are designated by like numerals as used before towhich 100 is added.

The pseudo-reaction imparting means 131 includes a reaction piston 138disposed forwardly of a valve body 103. The reaction piston 138 includesa portion 138 a of a reduced diameter located at the right end, a pairof arms 138 b extending radially outward from the left end of theportion 138 a, and a tubular portion 138 c extending from the distalends of arms 138 b. The portion 138 a is slidably fitted into the valvebody 103, from the front side, while maintaining a hermetic sealtherebetween. When the brake booster is inoperative, a valve plunger 111is substantially in abutment against the portion 138 a.

Formed in the right end of an output shaft 124 is a recess 124 a, inwhich an annular projection 103 a formed at the right end of the valvebody 103 is fitted while the right end face of the output shaft is heldin abutment against the valve body 103, thus connecting the both membersintegrally or in a manner to prevent a relative displacementtherebetween.

The right end of the output shaft 124 (FIG. 2) is formed with slits 124b which extend axially thereof, thus allowing the arms 138 b of thereaction piston 138 to extend therethrough. In this manner, an axialdisplacement of the reaction piston 138 with respect to the output shaft124 is permitted. A spring 139 is provided between the tubular portion138 c of the reaction piston 138 and a front shell 101 to produce apseudo-reaction.

In other respects, the arrangement is similar to the first embodiment.In the present embodiment, when the booster is inoperative as shown, orwhen a key member 122 abuts against the inner surface of a rear shell102 to maintain the key member 122 and a valve plunger 111 at anadvanced position relative to the valve body 103, the valve plunger 111abuts the against the portion 138 a of the reaction piston 138. When thebrake pedal is depressed under this condition to drive an input shaft121 and the valve plunger 111 forward, the spring 139 will becompressed, whereby a pseudo-reaction having a magnitude which isproportional to the compression of the spring is transmitted to a driverthrough the reaction piston 138, the valve plunger 111 and the inputshaft 121.

On the other hand, the braking reaction which is applied to the outputshaft 124 is entirely transmitted to the valve body 103, and no portionthereof is transmitted to the valve plunger 111, and hence to the brakepedal.

As a consequence, an excellent brake feeling which is comparable to theprior art is assured for the driver, and a reliable quick brakingoperation is assured for a physically weaker driver such as an agedperson or woman.

When the brake booster is released, or when the brake pedal is released,the retracting movement of the reaction piston 138 is limited byabutment against the valve body 103, but the valve plunger 111 isallowed to retract relative to the valve body 103 until its retractingmovement is limited by the key member 122, whereby the valve plunger 111b will be spaced from the portion 138 a having a reduced diameter of thereaction piston 138.

When the pressure within the variable pressure chamber 107 is reduced asa result of switching the flow path of the valve mechanism 109, a powerpiston 104 and the valve body 103 retract under the influence of areturn spring 128. The retracting movement will be limited initiallywhen the key member 122 abuts against the inner surface of the rearshell 102, but the valve body 103 continues to retract until it abutsagainst the key member 122 where it comes to a stop.

Under this condition, the key member 122 and the valve plunger 111 whichis associated therewith will be maintained at an advanced positionrelative to the valve body 103, and return to their inoperativeconditions as the valve plunger 111 moves forward when the distal endthereof abuts against the portion 138 a of the reaction piston 138.

Third Embodiment

FIG. 3 shows a third embodiment of the invention where it is to be notedthat corresponding parts or members as shown in FIG. 1 are designated bylike numerals as used before to which 200 is added.

In the present embodiment, a pseudo-reaction imparting means 231comprises a spring 243 disposed between a brake pedal 241 and acarrosserie 242. Specifically, a clevis 244 is mounted on one end of aninput shaft 221, and the clevis 244 and the brake pedal 241 areconnected together by a clevis pin 245. One end of the spring 243 isengaged with the pin 245. It is to be noted that the distal end of thebrake pedal 241 is connected to a carrosserie 242 in a rockable mannerby a connecting pin 246. Rather than utilizing the clevis pin 245, thespring 243 may be directly connected to the brake pedal 241 or to theinput shaft 241. It will be apparent that the use of suchpseudo-imparting means 231 is effective to achieve the functioning asmentioned above.

It is to be noted that in the present embodiment, an opening in whichthe distal end of a valve plunger 211 is slidably fitted so as toprevent a braking reaction applied to an output shaft 224 from beingtransmitted to the valve plunger 211 is not a through-opening as shownin the described embodiments, but has its end located toward the outputshaft 224 blocked.

In the described embodiments, pseudo-reaction imparting means 31, 131and 231 utilize springs 32, 139 and 243 as elastic members, but the useof such springs is not essential, but an elastic member such as formedof rubber may also be used.

Fourth Embodiment

FIGS. 4 to 6 show a fourth embodiment of the invention in which a firstspring 336 and a second spring 342 constitute together a pseudo-reactionimparting means 331 of the present embodiment in contra-distinction tothe previous embodiments in which a single spring 32, 139 or 243 isused. It is to be understood that the essential parts of the brakebooster according to the fourth embodiment shown in FIG. 4 are similarin construction to that of a brake booster shown in the firstembodiment, and accordingly, corresponding parts or members aredesignated by like reference numerals as used in the first embodiment towhich 300 is added.

Referring to FIG. 5, a brake pedal 321 has its one end connected to acarrosserie 333 in rockable manner by means of a connecting pin 332. Afirst clevis 334 is mounted on one end of an input shaft 319, and aclevis pin 335 extends through the first clevis 334 and through thebrake pedal 321, thus connecting the input shaft 319 and the brake pedal321 together.

As shown in FIGS. 5 and 6, the first spring 336 having a reducedresilience is disposed between the clevis pin 335 and the carrosserie333, and the tension of the first spring 336 is effective to hold thebrake pedal 321 in abutment against a first stop 337 (FIG. 5 ) mountedon the carrosserie 333 normally. Rather than connecting it to the clevispin 335, the first spring 336 may be directly connected to the brakepedal 321, to the first clevis 334 or to the input shaft 319.

The first clevis 334 is provided within a second clevis 341 so as to bereciprocable axially of the input shaft 319, and the opposite ends ofthe clevis pin 335 are engaged with elongate slots 341 a formed axiallyof the input shaft 319 in the second clevis 341, whereby a relativemovement between the first clevis 334 and the second clevis 341 ispossible within an extent of the length of the elongate slots 341 a. Thesecond spring 342 having a resilience which is greater than theresilience of the first spring 336 is disposed between the second clevis341 and the carrosserie 333 and extends in a parallel relationship withthe first spring 336. The tension of the spring 342 is effective tomaintain the second clevis 341 normally in abutment against a secondstop 343 provided on the carrosserie 333. Under this condition, theclevis pin 335 lies toward the rear end of the elongate slots 341 a.

With the described arrangement, when the brake pedal 321 is depressed todrive the input shaft 319 and a valve plunger 311 forward, a flow pathin a valve mechanism 309 is switched to introduce the atmosphere into avariable pressure chamber 307 in the similar manner as in a conventionalbooster, whereby a pressure differential between a constant pressurechamber 306 and the variable pressure chamber 307 drives a power piston304 and a valve body 303 forward to thereby drive the output shaft 324forward, causing a braking liquid pressure to be developed in a mastercylinder. A braking reaction which results from the braking liquidpressure is entirely transmitted to the valve body 303 through theoutput shaft 324, an elastic member 325 and a spacer 326, but nottransmitted to the valve plunger 311.

On the other hand, when the brake pedal 321 is depressed, it initiallycauses the first spring 336, and then the second spring 342 to betensioned, whereby a pseudo-reaction which depends on such tension isproduced on the brake pedal 321.

Specifically, when the brake pedal 321 is depressed, it is initiallysubject to the resilience of the spring 336. As the brake pedal 321 isdepressed further, and the clevis pin 335 which is integral with thebrake pedal 321 moves to the forward end of the elongate slots 341 aformed in the second clevis 341, the clevis pin 335 causes the secondclevis 341 to be driven forward against the resilience of the secondspring 342, whereupon the resilience of both the first spring 336 andthe second spring 342 which are disposed parallel to each other actssimultaneously upon the brake pedal 321.

A resulting relationship between the stroke of the brake pedal 321 and aforce of depression is graphically shown in FIG. 7, where acharacteristic represented by a straight line A is produced by the firstspring 336 and has a slope which is determined by a spring constant k₁of the first spring 336, and an intercept B with the abscissa isdetermined by the preset load upon the first spring 336. It will be seenfrom this characteristic represented by the straight line A that becausethe resilience of the first spring 336 is chosen to be small, the strokeincreases rapidly in response to a small increase in the force ofdepression of the brake pedal 321 during the initial phase of thebraking operation.

A point C shown in FIG. 7 represents a beginning point when theresilience of both the first spring 336 and the second spring 342 actsupon the brake pedal 321 simultaneously. A characteristic represented bya straight line D is produced by a combination of the first spring 336and the second spring 342, and has a slope which is determined by acombined effect of a spring constant k₁ of the first spring 336 and aspring constant k₂ of the second spring constant 342. The combinedspring constant k is represented as: k=k₁+k₂. Because the resilience ofthe second spring 342 is chosen to be greater than that of the firstspring 336, subsequent to the point C, an increase in the stroke can bereduced in response to a relatively large increase in the force ofdepression.

It is to be understood that a more excellent brake feeling can beobtained by a combination of the straight lines A and D mentioned aboverather than determining the relationship between the stroke and theforce of the depression in accordance with a single straight line or asingle spring.

It is to be understood that the pseudo-reaction imparting means 331imparts a pseudo-reaction to the brake pedal 321, which remainssubstantially unchanged between the quick and the normal brakingoperation, avoiding the occurrence of an abnormally high reaction duringa quick braking operation as experienced with a conventionalarrangement. Accordingly, a high braking output can be obtained with arelatively small force of depression during a quick braking operation,and thus a reliable quick operation is assured for a powerless driversuch as an aged person or woman.

Fifth Embodiment

FIG. 8 shows a fifth embodiment of the invention in which a pair ofsprings are disposed in series rather than in a parallel relationship asshown in the fourth embodiment. As before, parts and memberscorresponding to those shown in FIG. 1 are designated by like numeralsas used in FIG. 1 to which 400 is added.

In the present embodiment, a pseudo-reaction imparting means 431comprises a pair of a first spring 436 and a second spring 442 which aredisposed in series between a brake pedal 421 and a carrosserie 433 andwhich are connected to each other. A stop plate 451 is provided at thejuncture between the both springs 436, 442, and the stop plate isdisposed for abutment against a stop 452 mounted on the carrosserie 433.

In the inoperative condition, the stop plate 451 is spaced a givendistance from the stop 452, and when the brake pedal 421 is depressedunder this condition, the both springs 436, 442 are tensioned to causethe stop plate 451 to abut against the stop 452, whereupon only thefirst spring 436, which is located nearer to the brake pedal 421 will betensioned. A combined spring constant k when a pair of springs which areconnected in series are tensioned is given as k=k₁·k₂/(k₁+k₂) where k₁and k₂ represent the spring constant of the respective springs. It willbe apparent from this equation that the combined spring constant k willbe reduced than any one of the spring constants k₁ and k₂ of therespective springs, and such a combined spring constant k having areduced magnitude can be used to achieve the characteristic asrepresented by the straight line A shown in FIG. 7.

On the other hand, subsequent to the abutment of the stop plate 451against the stop 452, only the first spring 436 will be tensioned, thusproducing the characteristic as represented by the straight line D shownin FIG. 7 in accordance with the spring constant k₁.

In each of the described embodiments, each single spring 336, 342, 436,442 may comprise a plurality of springs which are connected together inseries or in parallel, thereby providing a required combined springconstant in a flexible manner.

Sixth Embodiment

FIGS. 9 and 10 show a sixth embodiment of the invention in which apseudo-reaction imparting means 531 comprises a first spring whichutilizes the valve return spring 20 shown in FIG. 1 and a second springwhich comprises a torque spring 542. As before, parts and memberscorresponding to those shown in FIG. 1 are designated by like referencenumerals as used in FIG. 1 to which 500 is added.

The torque spring 542 is disposed in surrounding relationship with aconnecting pin 532 which connects a brake pedal 521 to a carrosserie533. One end 542 a of the torque spring abuts against a stop 555 mountedon the brake pedal 521 while the other end 542 b is engaged with anarcuate groove 556 a formed in a bracket 556 which forms part of thecarrosserie 533. The arcuate groove 556 a is formed to be arcuate aboutthe connecting pin 532.

The brake pedal 521 is normally maintained such that the stop 555 is inabutment against a stop 557 which is mounted on the carrosserie 533.Under this condition, the other end 542 b of the torque spring 542 isspaced a given distance δ from the end of the arcuate groove 556 a.

When the brake pedal 521 is depressed under this condition, an inputshaft is driven forward against the resilience of the valve returnspring 20, and subsequently when the other end 542 b of the torquespring 542 abuts against the end of the arcuate groove 556 a, theresilience of the torque spring 542 becomes acting upon the brake pedal521.

Accordingly, a characteristic as represented by the straight line Ashown in FIG. 7 is achieved by the valve return spring 20 while acharacteristic as represented by the straight line D as shown in FIG. 7can be achieved by a combination of the valve return spring 20 and thetorque spring 542.

In the described embodiments, the brake pedal 241, 321, 421 or 521 isutilized as an operating rod, but the operating rod may also comprise amanual brake lever which is provided for a physically handicappedperson. It should be understood that the invention is also applicable toa clutch booster where a clutch pedal is used as an operating rod.

While the invention has been disclosed above in connection with severalembodiments thereof, it should be understood that these embodiments areillustrative only, and that a number of changes, modification andsubstitutions therein are possible without departing from the spirit andthe scope of the invention as defined by the appended claims.

What is claimed is:
 1. In a booster including a valve body slidablydisposed within a shell, a power piston integral with the valve body, aconstant pressure chamber and a variable pressure chamber definedadjacent opposite sides of the power piston, a valve mechanism disposedin the valve body, an input shaft connected to an operating rod fordriving a valve plunger, which forms the valve mechanism, back and forthto switch a flow path in the valve mechanism, and an output shaft havinga recess formed in a rear portion thereof and fitted on the valve bodyso that it is driven forward as the valve body is driven forward; areaction mechanism for the booster comprising a pseudo-reactionimparting means which prevents a reaction applied to the output shaftfrom being transmitted to the valve plunger when the booster isoperative and which imparts a pseudo-reaction which depends on a travelof the input shaft, the pseudo-reaction imparting means comprising atubular member secured to the shell, and an elastic member disposedbetween the tubular member and the input shaft.
 2. A reaction mechanismfor booster according to claim 1 in which the tubular member comprisesan outer tubular portion fitted over the outer periphery of a tubularportion formed in the shell, an inner tubular portion which is insertedinto the valve body, a connector portion which joins both said tubularportions together at their rear ends, and a flange which projectsradially inward from the front end of the inner tubular portion, and theelastic member being disposed between the flange and the input shaft. 3.A reaction mechanism for booster according to claim 2 in which thepseudo-reaction imparting means imparts a pseudo-reaction having areduced rate of increase to the input shaft when a travel of the inputshaft is small, and imparts a pseudo-reaction having an increased rateof increase to the input shaft when a travel of the input shaft ishigher.
 4. A reaction mechanism for booster according to claim 2 inwhich the elastic member comprises a spring.
 5. A reaction mechanism forbooster according to claim 1 in which the elastic member comprises aspring.
 6. In a booster including a valve body slidably disposed withina shell, a power piston mounted on the valve body, a constant pressurechamber and a variable pressure chamber defined adjacent opposite sidesof the power piston, a valve mechanism disposed in the valve body, aninput shaft connected to an operating rod for driving a valve plunger,which forms the valve mechanism, back and forth to switch a flow path inthe valve mechanism, and an output shaft driven forward as the valvebody is driven forward; a reaction mechanism for the booster comprisinga pseudo-reaction imparting means which imparts a pseudo-reaction havinga reduced rate of increase to the input shaft when a travel of the inputshaft is small and an increased rate of increase to the input shaft whena travel of the input shaft is higher and prevents a reaction applied tothe output shaft from being transmitted to the valve plunger when thebooster is operative, the pseudo-reaction imparting means comprising atubular member secured to the shell and an elastic member disposedbetween the tubular member and the input shaft, and the operating rodcomprising a pedal connected to a carrossiere in a rockable manner witha connecting pin, a torque spring disposed around the connecting pinsuch that one end thereof abuts against the pedal while the other endabuts against the carrosserie, either one of the ends being spaced agiven distance from its mating member and abutting against such memberafter the pedal has been depressed through a given stroke.